Carbon-containing interfacial layer for phase-change memory

ABSTRACT

A phase-change memory cell may be formed with a carbon-containing interfacial layer that heats a phase-change material. By forming the phase-change material in contact, in one embodiment, with the carbon containing interfacial layer, the amount of heat that may be applied to the phase-change material, at a given current and temperature, may be increased. In some embodiments, the performance of the interfacial layer at high temperatures may be improved by using a wide band gap semiconductor material such as silicon carbide.

This is a divisional of prior application Ser. No. 09/975,272, filedOct. 11, 2001, which issued as U.S. Pat. No. 6,566,700.

BACKGROUND

This invention relates generally to memories that use phase-changematerials.

Phase-change materials may exhibit at least two different states. Thestates may be called the amorphous and crystalline states. Transitionsbetween these states may be selectively initiated. The states may bedistinguished because the amorphous state generally exhibits higherresistivity than the crystalline state. The amorphous state involves amore disordered atomic structure and the crystalline state involves amore ordered atomic structure. Generally, any phase-change material maybe utilized; however, in some embodiments, thin-film chalcogenide alloymaterials may be particularly suitable.

The phase-change may be induced reversibly. Therefore, the memory maychange from the amorphous to the crystalline state and may revert backto the amorphous state thereafter or vice versa. In effect, each memorycell may be thought of as a programmable resistor, which reversiblychanges between higher and lower resistance states.

In some situations, the cell may have a large number of states. That is,because each state may be distinguished by its resistance, a number ofresistance determined states may be possible allowing the storage ofmultiple bits of data in a single cell.

A variety of phase-change alloys are known. Generally, chalcogenidealloys contain one or more elements from column VI of the periodictable. One particularly suitable group of alloys are GeSbTe alloys.

A phase-change material may be formed within a passage or pore definedthrough a dielectric material. The phase-change material may be coupledto contacts on either end of the passage.

The phase-change may be induced by heating the phase-change material. Insome embodiments of phase-change memories, a current is applied througha lower electrode that has sufficient resistivity or othercharacteristics to heat the phase-change material and to induce theappropriate phase change. In some embodiments, the lower electrode mayproduce temperatures on the order of 600° C.

One problem with existing electrode arrangements is that the higher thetemperature, the lower the resistivity of the material. Thus, as thelower electrode is heating up in order to induce the phase change, itprogressively becomes less resistive, thereby decreasing the amount ofheat that is generated.

Thus, there is a need for a controllable way to provide sufficientresistance proximate to the phase-change material even at elevatedtemperatures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a greatly enlarged, cross-sectional view in accordance withone embodiment of the present invention;

FIG. 2 is a greatly enlarged, cross-sectional view of an early stage offabrication of the device shown in FIG. 1 in accordance with oneembodiment of the present invention;

FIG. 3 is a greatly enlarged, cross-sectional view of the embodimentshown in FIG. 2 at a subsequent stage of manufacturing in accordancewith one embodiment of the present invention;

FIG. 4 is a greatly enlarged, cross-sectional view of the embodimentshown in FIG. 3 at the subsequent stage of manufacturing in accordancewith one embodiment of the present invention;

FIG. 5 is a greatly enlarged, cross-sectional view of the embodiment ofFIG. 4 at a subsequent stage of manufacturing in accordance with oneembodiment of the present invention;

FIG. 6 is a greatly enlarged, cross-sectional view of a subsequent stageof manufacturing in accordance with one embodiment of the presentinvention;

FIG. 7 is an enlarged, cross-sectional view of still a subsequent stageof manufacturing in accordance with one embodiment of the presentinvention; and

FIG. 8 is a schematic depiction of a system in accordance with oneembodiment of the present invention.

DETAILED DESCRIPTION

Referring to FIG. 1, a memory cell 10 may include a phase-changematerial layer 24. The phase-change material layer 24 may be sandwichedbetween an upper electrode 26 and a lower electrode 14. In oneembodiment, the lower electrode 14 may be cobalt silicide. However, thelower electrode 14 may be any conductive material. Similarly, the upperelectrode 26 may be any conductive material.

The lower electrode 14 may be defined over a semiconductor substrate 12.Over the lower electrode 14, outside the region including thephase-change material layer 24, may be an insulative material 16, suchas silicon dioxide or silicon nitride, as two examples. A buriedwordline (not shown) in the substrate 12 may apply signals and currentto the phase-change material 24 through the lower electrode 14.

A carbon-containing interfacial layer 20 may be positioned between thephase-change material layer 24 and the insulator 16. In one embodiment,a cylindrical sidewall spacer 22 may be defined within a tubular porethat is covered by the carbon-containing interfacial layer 20 and thephase-change material layer 24.

In one embodiment of the present invention, the carbon-containinginterfacial layer 20 may be formed of silicon carbide. Silicon carbide,in its single crystal form, is a wide band gap semiconductor withalternating hexagonal planes of silicon and carbon atoms. Siliconcarbide may be heated to 600° C. in operation and may have a resistivitythat does not significantly go down with increasing temperature.Therefore, silicon carbide is very effective for heating thephase-change material layer 24. Again, it is desirable to heat thephase-change material layer 24 to induce changes of the phase-changematerial layer 24 between the amorphous and crystalline states.

The interfacial layer 20 does not increase its conductivity withincreasing temperature to the same degree as other available materialssuch as cobalt silicide. The reduced resistivity at increasedtemperature makes conventional materials less than ideal as heatingelectrodes for the phase-change material layer 24. At relatively hightemperatures, such as 600° C., where the resistivity of other materialsdecreases, the effectiveness of the interfacial layer 20 as a heater toinduce phase changes is not substantially diminished.

Silicon carbide, in particular, is less prone to losing its resistivityat higher temperatures because it is a wide band gap material. Otherwide band gap materials include galium nitride and aluminum nitride.Other carbon containing materials that may be utilized as theinterfacial layer 20 in embodiments of the present invention may includesputtered carbon and diamond.

The interfacial layer 20 may be deposited, for example, by chemicalvapor deposition in the case of silicon carbide and by sputtering in thecase of diamond or carbon. Other layer forming techniques may beutilized as well.

In some embodiments, it may be desirable to dope the interfacial layer20 to increase its conductivity. In some embodiments, undoped siliconcarbide, for example, may have too high a resistivity, resulting ineither too high a temperature or too much voltage drop across theelectrodes 14 and 26. Thus, ion implantation, for example, may beutilized to dope the layer 20 with P-type or N-type impurities toimprove its conductivity after annealing.

In some embodiments of the present invention, a layer (not shown) may beprovided to improve the adhesion between the phase-change material layer24 and the carbon-containing interfacial layer 20. Suitable adhesionpromoting layers may include any conductive materials includingtitanium, titanium nitride and Tungsten, as a few examples.

Referring to FIG. 2, a semiconductor substrate 12 may be covered withthe lower electrode 14 in one embodiment. The electrode 14 may then becovered by an insulator 16 and a suitable pore 18 formed through theinsulator 16.

The resulting structure may be blanket deposited, for example usingchemical vapor deposition, with the carbon-containing interfacial layer20 as shown in FIG. 3. Thereafter, in some embodiments, thecarbon-containing interfacial layer 20 may be subjected to an ionimplantation, as shown in FIG. 4, to increase its conductivity and todecrease its resistivity after annealing.

As shown in FIG. 5, a spacer material 22 may be deposited over the layer20. The spacer material 22 may, in one embodiment, be a chemical vapordeposited oxide. The oxide material 22 may then be subjected to ananisotropic etch to form the cylindrical sidewall spacer 22, shown inFIG. 6, in the pore 18.

Turning to FIG. 7, in one embodiment, the phase-change material layer 24may be formed into the pore 18 and specifically into the region definedby the sidewall spacer 22 so as to contact the layer 20. An upperelectrode 26 may be deposited over the phase-change material 24. Then,the electrode 26 and the phase-change material 24 may be patterned andetched to form the structure shown in FIG. 1.

Through the use of a carbon-containing interfacial layer 20, theresistivity of the phase-change material heater may be substantiallyincreased while at the same time improving the heating performance ofthe heater at high temperatures. The heater effectively includes theseries combination of the lower electrode 14 and the carbon-containinginterfacial layer 20. However, a series resistive combination isdominated by the element with the higher resistance, which may be thecarbon-containing interfacial layer 20 in some embodiments. As a result,the resistance of the series combination of layers 20 and 14 may bedominated by the resistance of the interfacial layer 20.

Referring to FIG. 8, the memory cell shown in FIG. 1 may be replicatedto form a memory array including a large number of cells. That memorymay be utilized as a memory of a wide variety of processor-based systemssuch as the system 40 shown in FIG. 8. For example, the memory may beutilized as the system memory or other memory in a variety of personalcomputer products such as laptop products or desk top products orservers. Similarly, the memory may be utilized in a variety ofprocessor-based appliances. Likewise, it may be used as memory inprocessor-based telephones including cellular telephones.

In general, the use of the phase-change memory may be advantageous in anumber of embodiments in terms of lower cost and/or better performance.Referring to FIG. 8, the memory 48, formed according to the principlesdescribed herein, may act as a system memory. The memory 48 may becoupled to a interface 44, for instance, which in turn is coupledbetween a processor 42, a display 46 and a bus 50. The bus 50 in such anembodiment is coupled to an interface 52 that in turn is coupled toanother bus 54.

The bus 54 may be coupled to a basic input/output system (BIOS) memory62 and to a serial input/output (SIO) device 56. The device 56 may becoupled to a mouse 58 and a keyboard 60, for example. Of course, thearchitecture shown in FIG. 8 is only an example of a potentialarchitecture that may include the memory 48 using the phase-changematerial.

While the present invention has been described with respect to a limitednumber of embodiments, those skilled in the art will appreciate numerousmodifications and variations therefrom. It is intended that the appendedclaims cover all such modifications and variations as fall within thetrue spirit and scope of this present invention.

1. A method comprising: forming a carbon-containing interfacial layer ona semiconductor; increasing the electrical conductivity of saidcarbon-containing interfacial layer; and forming a phase-change materialover said carbon-containing interfacial layer.
 2. The method of claim 1wherein forming a carbon-containing interfacial layer on a semiconductorincludes forming said interfacial layer over a conductive layer formedover a semiconductor.
 3. The method of claim 1 wherein forming acarbon-containing interfacial layer includes forming a layer including awide band gap semiconductor material.
 4. The method of claim 3 whereinforming a carbon-containing interfacial layer includes forming a siliconcarbide layer.
 5. The method of claim 4 further including doping saidsilicon carbide layer.
 6. The method of claim 5 further including dopingsaid silicon carbide layer using ion implantation.
 7. The method ofclaim 1 including forming a pore through an insulator, depositing saidcarbon-containing interfacial layer over said semiconductor and in saidpore.
 8. The method of claim 7 including depositing the phase-changematerial over the carbon-containing interfacial layer in said pore. 9.The method of claim 8 including forming a sidewall spacer between saidinterfacial layer and said phase-change material.
 10. The method ofclaim 1 wherein forming a phase-change material includes depositing achalcogenide layer over said interfacial layer.